1. Field of the Invention
This invention relates to materials which are highly resistant to hot, concentrated sulfuric acid and 0-10% by weight oleum.
2. Description of the Prior Art
The resistance of materials to the corrosive effect of hot, concentrated sulfuric acid is widely discussed in the literature.
On account of the increase in the solubility of lead sulfate with increasing sulfuric acid concentration, lead and its alloys can only be used at H.sub.2 SO.sub.4 concentrations of up to 78% and at temperatures of only up to 110.degree. C. (Ullmanns Encyclopadie der technischen Chemie, 4th Edition, Vol. 21 (1982), page 157).
Unalloyed steel can be used in 68 to 99% sulfuric acids up to 70.degree. C., although erosion rates of up to 1.3 mm/a can be expected (G. Nelson, Corrosion Data Survey, Shell Development Co., San Francisco, 1950, pp. ZT-102A). In the concentration range of 99 to 100% H.sub.2 SO.sub.4 there is a considerable reduction in the resistance of unalloyed steel. Relatively high flow rates should be avoided in the case of unalloyed steel ((Ullmann, loc. cit.; Z. f. Werkst.-Techn. 4 (1973), pp. 169/186; R. J. Borges, Corrosion/87, Paper No. 23, NACE, Houston, Tex., 1987).
Cast irons alloyed with chromium or copper are corrosion-resistant in 90 to 99% sulfuric acid at temperatures of up to about 120.degree. C. (Ullmann, loc. cit.), although corrosion is again dependent on the flow rate (Z. f. Werkst.-Techn., loc. cit.). The iron-silicon cast material containing 14 to 18% Si shows very good corrosion resistance in wide concentration and temperature ranges (Ullmann, loc. cit.); however, a major disadvantage of this special cast iron is that it is hard and brittle (R. J. Borges, Corrosion/87, loc. cit.; Ullmann, 4th Edition, Vol. 3 (1973), page 21). Stainless austenitic standard steels, for example Werkst.-Nr. (material No.) 1.4571, are used in concentrated sulfuric acids up to temperatures of 85.degree. C. The erosion rates increase steeply with increasing temperature. Erosion rates of the order of 1 mm/a can be expected at only 150.degree. C. (Z. f. Werkst.-Techn. 8 (1977), pp. 362/370 and 410/417), corrosion again showing pronounced dependence on the flow rate.
The use of nickel base alloys does not afford any advantages. In plate-type heat exchangers of NiMo16Cr15W, Werkst-Nr. (material No.) 2.4819 (type Hastelloy alloy C-276), which are used for cooling concentrated sulfuric acid, the product temperature is limited to 95.degree. C. (N. Sridhar, Materials Performance, March 1988, pp. 40/46).
Accordingly, there has been no shortage of proposals to improve resistance to sulfuric acid by alloying measures. Thus, the stainless austenitic steel X 1 CrNiSi 18 15 containing 3.7 to 4.3% Si, Werkst.-Nr. (material No.) 1.4361, shows considerably higher resistance than Werks.-Nr. (material No.) 1.4571 in, for example, 98.5% sulfuric acid at 150.degree. to 200.degree. C. (Ullmann, Vol. 3, page 21); the dependence of corrosion on the flow rate is very low (Z. f. Werkst. Techn. 8 (1977), pp. 362/370 and 410/417; M. Renner and R. Kirchheiner, "Korrosionbestandigkeit von hochlegierten nichtrostenden Sonderstahlen in stark oxidierenden Medien (Corrosion resistance of highly alloyed stainless special steels in highly oxidizing media)". Paper presented at the Seminar "Nickelwerkstoffe und hochlegierte Sonderstahle (Nickel materials and highly alloyed special steels)", Esslingen, 7th/8th Apr., 1986). The corrosion resistance of austenitic stainless steels in hot 85%, preferably 90%, sulfuric acids can be improved within certain limits by further increasing the Si content to 4.5 to 5.8% (U.S. Pat. No. 4,543,244; DE-OS 33 20 527). On account of the pronounced dependence of corrosion on temperature, however, a special steel such as this would be unsuitable for use at relatively high temperatures. The following erosion rates were determined for a fully austenitic stainless steel having the composition 17.5% Cr, 17.5% Ni, 5.3% Si, rest essentially iron, in 98.2% sulfuric acid (cf. U.S. Pat. No. 4,543,244 and DE-OS 33 20 527):
125.degree. C.--0.1 mm/a PA1 135.degree. C.--0.8 mm/a PA1 145.degree. C.--1.6 mm/a;
a corrosion rate of 0.25 mm/a was determined in 93.5% H.sub.2 SO.sub.4 at 85.degree. C. To reduce corrosion, the plants could be given anodic protection. Under these conditions, however, the erosion rate in 93.5% H.sub.2 SO.sub.4 at 200.degree. C. is still 1.1 mm/a.
In addition, hardenable nickel base alloys containing 2 to 4% Si have been proposed for handling hot, at least 65% sulfuric acid (DE-PS 21 54 126). However, the erosion rates in sulfuric acid heated to 120.degree. C. of approximately 0.6 mm/a are extremely high. Erosion rates of 0.25 mm/a in 98% H.sub.2 SO.sub.4 heated to 140.degree. C. are cited for another hardenable nickel base alloy unaffected by flow (R. J. Borges, Corrosion/87, loc. cit.).
By contrast, an austenitic stainless steel containing 17% Cr, 16% Ni, 3.7% Si and 2.3% Mo can only be used in cold sulfuric acids at concentrations below 10% and above 80% (Publication No. 235 of the CAFL: Uranus, rost- und saurebestandige Stahle fur schwierige Korrosionsprobleme (Uranus, stainless acid-resistant steels for difficult corrosion problems), p. 37). According to GB-PS 1,534,926, austenitic chrome-nickel-copper steels optionally alloyed with molybdenum must contain at least 4.1% or 4.7% silicon to guarantee high resistance to corrosion in 96.5% H.sub.2 SO.sub.4 heated to 110.degree. C. The same applies to iron-chrome-nickel-cobalt-silicon alloys in 99% H.sub.2 SO.sub.4 heated to 130.degree. C. (N. Sridhar, loc. cit.).
Finally, iron-chrome-nickel alloys containing 4 to 6% PG,5 silicon have been described in the literature, their delta-ferrite content being limited to 5 to 10% so that no coherent delta-ferrite network can be formed (D. J. Chronister and T. C. Spence, Corrosion 85, Paper 305, NACE, Boston/Mass., March 1985). A network such as this can be expected beyond delta-ferrite contents of 10%. The erosion rates of an alloy containing 4.8% Si described by D. J. Chronister et al. in 95% H.sub.2 SO.sub.4 heated to 110.degree. C. are initially relatively low (0.4 mm/a), but increase rapidly to 2.4 mm/a in the event of prolonged exposure. Alloys containing 5 to 5.2% Si showed corrosion rates of 0.11 to 0.56 mm/a under these conditions. Erosion rates of the order of 0.1 mm/a are only observed at an Si content of 5.6%. However, if the temperature of the 95% H.sub.2 SO.sub.4 is increased to 130.degree. C., increasing erosion rates of 0.6 mm/a in the first test phase (48 h) and of as high as 1.24 mm/a in the second phase are again observed at an Si content of 5.6%. At an Si content of 5.9%, the erosion rates reach 0.45 to 0.54 mm/a.